Fourier transform infrared (FT-IR) spectroscopic imaging is a highly versatile technique that can be applied to a wide range
of systems. This article summarizes some of the recent efforts developing applications of FT-IR imaging for microfluidics.
The main advantage of FT-IR imaging compared to traditional imaging methods is that it is a label-free imaging technique.
There is no need to develop tags or labels, multiple components are simultaneously traced, and images can be taken without
disturbing the sample. All of these advantages are accompanied with a near-video frame rate acquisition speed. Different approaches
to obtain FT-IR images (transmission and attenuated total reflection mode) of microfluidic devices are discussed including
novel ways to create microfluidic devices.
Microfluidic technology is a powerful tool that has a wide range of applications in chemical and biological analysis (1).
The improved heat and mass transfer in microfabricated systems in comparison to traditional processes provides the opportunity
to increase control over the yield, the speed of the turnover of experiments for high-throughput studies, and reduce the amount
of precious reagents used (2). Knowing the chemical composition at a specific point in the microfabricated device can aid
in the design and optimization of these devices (3,4).
Detection in microfabricated devices often relies on additional tracers or tags to visualize the existence and the distribution
of particular components. One of the most widely used methods is confocal fluorescence microscopy. The advantage of fluorescence
is that it is highly sensitive (5) and can often achieve sufficient spatial resolution. However, finding a suitable fluorescence
agent and photobleaching are some of the remaining challenges to overcome. The tracer also has to be inert enough that it
will not decompose or interfere with the process of interest (for example, chemical reactions and diffusion). Raman, surface-enhanced
Raman spectroscopy (SERS), and coherent anti-Stokes Raman scattering (CARS) have been shown as promising label-free detection
methods in microfluidic systems (6–10). In this article, we discuss some recent studies that have demonstrated that Fourier
transform infrared (FT-IR) spectroscopic imaging can be a powerful detection tool, as was proposed earlier (11), to extract
spatially resolved rich chemical information from microfluidic devices in a label-free manner.
FT-IR Spectroscopic Imaging
FT-IR imaging has been used as a highly versatile analytical method, providing spatially resolved, chemically specific information
for studying multicomponent systems (12). Recently, conventional FT-IR microscopy using a single-element detector was applied
to analyze fast reactions in microfluidic flows where spectral information from a specific location in a channel was obtained
(13). FT-IR spectroscopic imaging combines the benefits of imaging and spectroscopy providing chemical maps of studied samples.
The chemical specificity comes from the intrinsic molecular vibrations, revealed by spectral bands, while spatial information
is collected from the focal plane array (FPA) detectors. An FPA detector comprises thousands of detector pixels, each collecting
an infrared spectrum so that thousands of infrared spectra are collected simultaneously in a single imaging measurement. This
approach reduces the time required to collect all the spectral data when compared to point-to-point mapping using a single
element detector with apertures (14).
Characteristic spectral bands can then be used as markers for specific components, which allows their distribution within
the imaged area to be revealed. Multivariate methods are also available to separate components with similar spectral features
to generate representative maps for each component. A full mid-IR spectrum (4000–900 cm-1) is collected from each detector pixel, and multiple components can be simultaneously tracked in one single imaging measurement
either using univariate or multivariate approaches. This multicomponent imaging feature is a significant advantage compared
to ordinary fluorescence measurements in which only one component is traced at a time. The absorbance of a spectral band is
directly correlated to the concentration of the component so that results obtained can be quantified.
Because all spectra are collected simultaneously, FT-IR imaging using FPA detectors is suitable for studying dynamic systems
such as diffusion and dissolution (15–20) and has great potential for high-throughput applications, especially when combined
with the attenuated total reflection (ATR) sampling method (11,21).